Melt butter in a large pot over medium-high heat. Add sliced onions and sugar, stirring frequently. Cook for 20-25 minutes until onions are deeply golden brown and caramelized.

Add minced garlic and cook for 1 minute until fragrant. Pour in white wine and scrape up any browned bits from the bottom of the pot. Cook for 2-3 minutes until wine reduces by half.

Add beef broth, thyme sprigs, salt, and pepper. Bring to a boil, then reduce heat and simmer for 10 minutes. Remove thyme sprigs.

While soup simmers, toast baguette slices in the oven at 400°F for 5 minutes until lightly golden and crispy.

Preheat broiler. Ladle soup into oven-safe bowls. Top each with 2 slices of toasted bread and generously cover with shredded gruyère cheese.

Place bowls on a baking sheet and broil for 2-3 minutes until cheese is melted, bubbly, and golden brown. Watch carefully to prevent burning.

Carefully remove bowls from oven (they will be very hot). Let cool for 2-3 minutes before serving.